There has long been a need for a relatively inexpensive, strong, high performance, permanent magnet. Such high performance permanent magnets would be characterized by relatively high magnetic parameters, e.g. coercive force (H.sub.c) or coercivity, remanent magnetization or remanence, and maximum energy product. Much inventive effort has gone into the development of high performance permanent magnets satisfying these criteria. Most of this effort has gone into development of the transition metal-rare earth-boron type system, the hard magnetic materials having a tetragonal crystal structure with a P4.sub.2 /mnm space group, exemplified by the Fe.sub.14 Nd.sub.2 B-type materials.
An ideal high-performance permanent magnet should exhibit a square magnetic hysteresis loop. That is, upon application of an applied magnetic field H greater than the coercive force Hc, all of the microscopic magnetic moments should align parallel to the direction of the applied force to achieve the saturation magnetization Ms. Moreover, this alignment must be retained not only for H=0 (the remanent magnetization Mr), but also for a reverse applied magnetic force of magnetude less than Hc. This would correspond to a maximum magnetic energy product (the maximum negative value of BH) of (Mr.sup.2 /4). Unfortunately, this ideal situation is at best metastable with respect to the formation of magnetic domains in other directions, which act to reduce Mr and (BH).sub.max.
E. C. Stoner and E. V. Wohlfarth, Phil. Trans. Royal Soc. (London), A. 240, 599 (1948) have calculated the hysteresis loop for permanent magnets with various orientations of the "easy axis of magnetization," that is, the c axis, with respect to the direction of an arbitrary applied magnetic field, that is, z. For an ideal array of randomly oriented non-interacting uniformly magnetized particles, i.e., an isotropic array, there is no dependence of the hysteresis loop on the direction of the applied field. The maximum theoretical value of the energy product of such a loop is dependent on M.sub.s and H.sub.c. If M.sub.s is chosen to equal 16 kilogauss and H.sub.c is chosen to be much greater than M.sub.s, then the maximum energy product is less then 16 megagaussoersteds. This is consistent with the observations of the prior art.
Contrary to the limited but negative teachings of the prior art, we have been able to utilize interactions between crystallites to achieve enhanced magnetic parameters in bulk solid materials, as described in our commonly assigned, copending U.S. application Ser. No. 893,516, filed Aug. 5, 1986 for Enhanced Remanence Permanent Magnetic Alloy And Bodies Thereof, incorporated herein by reference.
By "enhanced parameter" materials are meant ferromagnetic materials characterized by magnetic parameters, especially coercivity, remanence, and energy product, greater than those predicted by Stoner and Wohlfarth for non-interacting systems. These materials have a short range local order characterized by the mean crystallographic grain size, the crystallographic grain size range, and the crystallographic grain size distribution all being within narrow ranges and by the substantial absence of deleterious intergranular phases, as is more fully described in our commonly assigned, copending U.S. application Ser. No. 07/191,509, filed May 9, 1988 in the names of Richard Bergeron, R. William McCallum, Karen Canavan, John Keem, Alan M. Kadin, and Gregory B. Clemente for Enhanced Remanence Permanent Magnetic Alloy and Bodies Thereof, hereby incorporated by reference herein. The material or grain morphology, i.e., the grain size, grain size distribution, grain size range and the grain boundary phase distributions, are all correlated with the observed enhanced magnetic parameters and are believed to be associated with magnetic interactions between adjacent grains across grain boundaries.
The above applications, and their parent, U.S. application Ser. No. 816,778, also incorporated herein by reference, describe a class of permanent magnetic alloys which exhibit enhanced magnetic parameters, especially remanence and energy product, as measured in all spatial directions, that is, isotropically. The magnetic parameters are of a magnitude which the prior art teaches to be only attainable in one spatial direction, that is, anisotropically, and to be only attainable with aligned materials.
These enhanced parameter alloy materials of our commonly assigned copending applications, Ser. Nos. 816,778, now abandoned, 893,516, now abandoned, and 063,936, now U.S. Pat. No. 4,834,811 do not obey the Stoner and Wohlfarth assumptions of non-interacting particles. To the contrary, the individual particles or crystallites interact across grain boundaries. This interaction is consistent with ferromagnetic exchange type interaction presumably mediated by conduction electrons.
The enhanced parameter alloy is a substantially crystallographically unoriented, substantially magnetically isotropic alloy, with apparent interaction between adjacent crystallites. By substantially isotropic is meant a material having properties that are similar in all directions. Quantitatively, substantially isotropic materials are those materials where the average value of [Cos(theta)], defined above, is less than about 0.75 in all directions, where Cos (theta) is averaged over all the crystallites.
The enhanced parameter magnetic materials are permanent (hard) magnets, with isotropic maximum magnetic energy products greater than 15 megagaussoersteds, coercivities greater than about 8 kilooersteds at standard temperature (23.degree. C. to 27.degree. C.), and isotropic remanences greater than about 8 kilogauss, and preferably greater than above about 11 kilogauss.
The enhanced parameter magnetic material is composed of an assembly of small crystalline ferromagnetic grains. The grains are in intimate structural and metallic contact along their surfaces, i.e., along their grain boundaries. The degree of magnetic enhancement above the upper limits predicted by Stoner and Wohlfarth is determined by the grain morphology, the morphologies necessary for enhanced parameters include crystallite grain boundaries being sufficiently free of substantially continuous deleterious intergranular phases, the individual crystallites having the size, size distribution and size range of the grains relative to a characteristic size, R.sub.O.
While the interaction across grain boundaries and concommitant enhancement of properties has been quantitatively described in the above applications with respect to rare earth-transition metal-boron materials of tetragonal, P4.sub.2 /mnm crystallography, especially the Nd.sub.2 Fe.sub.14 B.sub.1 type materials having a characteristic size, Ro, of about 200 Angstroms, this is a general phenomenon applicable to other systems as well. The optimum characteristic size, R.sub.o, however, may be different in these other cases, as is described in our commonly assigned, copending U.S. application Ser. No. 893,516, incorporated herein by reference.
In one exemplification of our commonly assigned, copending U.S. applications Ser. Nos. 893,516 and 063,936 the magnetic alloy material is an alloy of iron, optionally with other transition metals, as cobalt, a rare earth metal or metals, boron, and a modifier. In another exemplification the magnetic alloy material is an alloy of a ferromagnetic transition metal as iron or cobalt, with an lanthanide, as samarium, and a modifier.
A modifier is an alloying element or elements added to a magnetic material which serve to improve the isotropic magnetic properties of the resultant material, when compared with the unmodified material, by an appropriate processing technique. Exemplary modifiers are silicon, aluminum, and mixtures thereof. It is possible that the modifier acts as a grain refining agent, providing a suitable distribution of crystallite sizes and morphologies to enhance interactions.
The amount of modifier is at a level, in combination with the quench parameters, to give the above described isotropic magnetic parameters.
The enhanced parameter magnetic alloy may be of the type [Rare Earth Metal(s)]-[Transition Metal(s)]-[Modifier(s)], for example [Sm]-[Fe, Co]-[Si, Al].
Another interacting alloy may be of the type [Rare Earth Metal(s)]-[Transition Metal(s)]-Boron-[modifier(s)], for example [Rare Earth Metal(s)]-[Fe, Co]-Boron-[modifier(s)], and [Rare Earth Metal(s)]-[Fe, Co, Mn]-Boron-[modifier(s)].
In one exemplification, the magnetic alloy material has the stoichiometry represented by: EQU (Fe, Co, Ni).sub.a (Nd, Pr).sub.b B.sub.c (Al, Si).sub.d,
exemplified by EQU Fe.sub.a (Nd, Pr).sub.b B.sub.c (Al, Si).sub.d,
where a, b, c, and d represent the atomic percentages of the components iron, rare earth metal or metals, boron, and silicon, respectively, in the alloy, as determined by energy dispersive spectroscopy (EDS) and wave length dispersive spectroscopy (WDS) in a scanning electron microscope. The values for these coefficients are: EQU a+b+c+d=100;
a is from 75 to 85; PA1 b is from 10 to 20, and especially from 11 to 13.5; PA1 c is from 5 to 10; PA1 and d is an effective amount, when combined with the particular solidification or solidification and heat treatment technique to provide a distribution of crystallite size and morphology capable of enhancement of magnetic parameters, e.g., from traces to 5.0.
The rare earth metal is a lanthanide chosen from neodymium and praseodymium, optionally with other lanthanides (one or more La, Ce, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb and Lu), Sc, Y, and mixtures thereof present. While various combinations of the rare earth metals may be used without departing from the concept of this invention, especially preferred rare earth metals are those that exhibit one or more of the following characteristics: (1) the number of f-shell electrons is neither 0 (as La), 7 (as Gd) or 14 (as Lu), (2) low molecular weight lanthanides, such as La, Ce, Pr, Nd, and Sm, (3) high magnetic moment lanthanides that couple ferromagnetically with iron, as Nd and Pr, or (4) relatively inexpensive lanthanides, as La, Ce, Pr, and Nd. Especially preferred are Nd and Pr. Various commerical and/or byproduct mischmetals may be used. Especially preferred mischmetals are those rich in Nd and/or Pr.
The preferred means of producing the above described, enhanced parameter, magnetic alloy having magnetic isotropy and the above short range order and/or crystallographic properties and dimensions is by melt spinning, i.e., rapidly solidifying and quenching molten alloy material onto a moving chill surface, e.g., a rotating chill surface means substantially as shown in commonly assigned, copending U.S. application Ser. No. 816,778, U.S. application Ser. No. 893,516, U.S. application Ser. No. 063,936, and U.S. application Ser. No. 07/191,626, filed May 9, 1988, now U.S. Pat. No. 4,867,785 in the names of John Keem, Jun Su Im, John Tyler, Richard Bergeron. Kevin Dennis, and David Hoeft for METHOD OF RAPIDLY QYENCHING A MOLTEN ALLOY TO FORM A SOLID ALLOY OF UNIFORM FINE GRAIN MICROSTRUCTURE, the disclosure of which is hereby incorporated by reference herein.
The quench parameters may be controlled to direct the solidification front, control its velocity, and control grain coarseness.
The alloy is quenched at an appropriate rate to result in morphological, crystallographic, atomic, and/or electronic structures and/or configurations that give rise to the novel enhanced magnetic parameters. The quench parameters are carefully controlled to produce flakes of a high fraction of an appropriate fine grained structure, which, together with the aforementioned modifier, results in the desired permanent magnet material.
These flakes are much larger then the characteristic crystallographic grain size, R.sub.o. A typical flake may contain at least 10.sup.8 grains of characteristic grain size R.sub.o.
Individual melt spun fragments are recovered as particulate flake product from the melt spinning process. Individual particles can also be obtained by the comminution of the ribbon fragments which are generally relatively brittle. The ribbon fractures, yielding smaller particles, e.g., flake like particles, or plate like individual particles.
As described above these enhanced magnetic parameter materials are synthesized in processes that require chemical and structural modifiers, and rapid solidification. The modifiers and rapid solidification synergistically interact to provide solidification and crystallization pathways that result in the short range local order and/or crystallographic grain sizes identified with enhanced parameters, e.g., remanence and energy product.
However, a significant problem is the effect of quench transients on the short range order, and, as a result, on the final magnetic properties. These transients may be of such short duration that a material is obtained having a distribution of short range local orders and/or crystallographic grain sizes and magnetic parameters in close proximity.
The short range local order of the enhanced parameter materials is a strong function of the instantaneous and time averaged local cooling rate (temperature change per unit time) and the instantaneous and time averaged thermal flux (energy per unit time per unit area). The solidification and crystallization processes occur with initial cooling rates of 100,000 to 1,000,000 degrees Celsius per second, and average temperature drops (temperature drop while on the chill surface divided by residence time on the chill surface) of 10,000 to 100,000 degrees Celsius per second. These cooling rates drive local instantaneous heat fluxes of hundreds of thousands of calories per square centimeter per second, and average heat fluxes of 10,000 to 100,000 calories per square centimeter per second. Within this cooling rate and heat flux regime, local, short duration upsets, transients, and excursions, as induction heating eddy currents, formation and passage of alloy-crucible reaction products (slags and oxides) through the crucible orifice, and even bubbles of inert propellant gases as argon, and the like, result in a particulate product containing a range of particle sizes, crystallographic grain sizes, and particle magnetic parameters ranging from overquenched to underquenched. When referring to the ribbon and/or flake product of the quench surface, the particle size correlated parameters are correlated primarily with the ribbon or flake thickness, and secondarily with the ribbon or flake width. by "particle size" we mean ribbon or flake thickness.
Short range local order including grain boundaries sufficiently free of substantially continuous deleterious intergranular phases, and/or the crystallographic grain size determines the magnetic parameters. Quench rate, i.e., cooling rate, and thermal flux, determine the short range local order. The ribbon or flake thickness, primarily, and width, secondarily, which we refer to as the ribbon or flake particle size is also correlated, to a first approximation, with the quench rate and the thermal flux. Thus, it is possible to effect a partial separation and an increased concentration of high parameter materials, including enhanced parameter materials, by particle size (i.e., thickness and width) classification alone. However, particle size classification alone results only in a separation of (1) a fraction enriched in over quenched and enhanced parameter materials from (2) a fraction enriched in under quenched material. This is a minimally efficient process, the resulting recovered product being slightly enriched in enhanced parameter material, but behaving macroscopically as overquenched material.
By "under quenched" materials are meant those materials having a preponderance of crystallographic grains larger than the grain sizes associated with enhanced magnetic parameters.
By "over quenched" materials are meant those materials having a preponderance of crystallographic grains smaller than the grain sizes associated with enhanced magnetic parameters. These are generally very low energy product materials. In some circumstances these overquenched materials can be heat treated to attain enhanced parameters.